It is known to improve the charging efficiency of an internal combustion engine by means of a supercharger, such as a mechanical supercharger. A mechanical supercharger of the type having screws, in which intake air is compressed, is advantageous, due to its high volumetric and heat insulation efficiency. Such a screw type supercharger, however, has been found to lose its pumping power due to internal compression when the internal combustion engine operates with a low, or only partial, load.
To decrease such a loss of pumping power, or pump loss, in a screw type supercharger of the type which is used in combination with turbochargers, a valve, slidable in a direction of the central axis of rotation of the screw type supercharger, has been used to variably change the size of an opening of an intake port of the screw type supercharger. An effective stroke of the screw type supercharger is thereby shortened when the internal combustion engine operates with a low load. Such a screw type supercharger is known from, for example, Japanese Unexamined Patent Application No. 63(1988)-170524.
In general, in an internal combustion engine with a supercharger, it is necessary to provide a relief mechanism for relieving the supercharger in order to restrict its maximum supercharging pressure. Previously, however, the provision of such a relief mechanism, in addition to a mechanism for regulating the effective stroke of the supercharger, has made the screw type supercharger complex in structure.